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With respect to
drawing air through the tower, there are three
types of cooling towers:
Natural draft, which utilizes buoyancy via a
tall chimney. Warm, moist air naturally rises
due to the density differential to the dry,
cooler outside air. Warm moist air is less dense
than drier air at the same pressure. This moist
air buoyancy produces a current of air through
the tower.
Mechanical draft, which uses power driven fan
motors to force or draw air through the tower.
Induced draft: A mechanical draft tower with a
fan at the discharge which pulls air through
tower. The fan induces hot moist air out the
discharge. This produces low entering and high
exiting air velocities, reducing the possibility
of recirculation in which discharged air flows
back into the air intake. This fan/fill
arrangement is also known as draw-through.
Forced draft: A mechanical draft tower with a
blower type fan at the intake. The fan forces air
into the tower, creating high entering and low
exiting air velocities. The low exiting velocity is
much more susceptible to recirculation. With the fan
on the air intake, the fan is more susceptible to
complications due to freezing conditions. Another
disadvantage is that a forced draft design typically
requires more motor horsepower than an equivalent
induced draft design. The forced draft benefit is
its ability to work with high static pressure. They
can be installed in more confined spaces and even in
some indoor situations. This fan/fill geometry is
also known as blow-through.
Fan assisted natural draft. A hybrid type that
appears like a natural draft though airflow is
assisted by a fan.
Hyperboloid (aka hyperbolic) cooling towers
have become the design standard for
all natural-draft cooling towers because of
their structural strength and minimum usage of
material. The hyperbolic form is popularly
associated with nuclear power plants. However,
this association is misleading, as the same kind
of cooling towers are often used at large
coal-fired power plants as well. Similarly, not
all nuclear power plants have cooling towers. |
